Refractory metal boride articles, for example, titanium diboride articles, have been proposed for use as cathodes in electrolytic aluminum reduction cells where the resistance of titanium diboride to chemical attack at very high temperatures in the corrosive environment, in addition to its high electrical conductivity, is very important. However, titanium diboride articles of moderate density, for example, 70-85 percent theoretical density, have been found to break down too quickly in the hostile environment of the aluminum cell to be of practical utility. The porosity of such articles occurs primarily at grain boundaries and thus represents imperfect bonding of one grain to another. It is believed that electrolyte in the aluminum cells is capable of penetrating between the poorly bonded grains along the boundaries, thereby eventually breaking down the structure of the refractory article. Titanium diboride articles of near theoretical density, e.g., 95-99 percent theoretical, are expected to have a significantly longer life time in aluminum reduction cells than less dense articles because the grains are better bonded and the boundaries are more resistant to penetration by electrolyte.
Copending U.S. Pat. application Ser. No. 546,835 filed Feb. 3, 1975, discloses that submicron Group IVb metal diboride powders, i.e., titanium, zirconium, and hafnium diboride powders, prepared as described therein, containing carbon or refractory metal carbide may be cold pressed and sintered to near theoretical density, i.e., at least 95 percent of theoretical. Such cold pressed and sintered articles have the dense matrix that is expected to be more resistant to corrosion and infiltration in very hostile environments such as that of an aluminum reduction cell. The disclosure of said application has been published as part of German Offenlegungsschrift 25 23 423.
Refractory Group IVb metal diboride, e.g., titanium diboride, articles of less than theoretical density which possess advantages manifested by articles of near theoretical density offer the further advantages of lower cost and lighter weight. However, articles of reduced overall density which have the greater part of their porosity at grain boundaries are subject to attack at such boundaries by an electrolyte such as is present in an aluminum reduction cell, and the life of the article consequently is shortened.